Short answer: BGP LOCAL_PREF is the cleanest attribute for choosing which exit point your autonomous system uses for outbound traffic. Higher LOCAL_PREF wins inside the local AS, it is carried across iBGP, and it is not advertised to external BGP neighbors because RFC 4271 defines LOCAL_PREF as a well-known discretionary attribute used inside an AS.[1]
If you remember one rule, remember this: use LOCAL_PREF to choose your outbound exit; do not expect it to directly force how the Internet sends traffic back to you.
